1. Field of the Invention
The present invention is applicable to an optical multiplexing circuit that multiplexes a plurality of optical signal together, and particularly, to an optical multiplexer applicable to an arrayed-waveguide grating type optical multiplexing circuit having a wavelength monitoring function for wavelength division multiplexing optical communication.
2. Description of the Related Art
FIG. 25 shows an arrayed-waveguide grating (AWG) type optical multiplexing and demultiplexing circuit utilizing multiple beams interference.
A structure of common arrayed-waveguide gratings will be described. Reference numeral 100 denotes a substrate composed of silicon or silica. A lower clad layer composed of a silicon oxide layer or the like is formed on the substrate 100. A silicon dioxide layer or the like is deposited on the lower clad layer and is doped with germanium as impurities to increase the refractive index thereof relative to that of the lower clad layer. This silicon layer is patterned to form a core layer. Further, an upper clad layer composed of a silicon oxide layer or the like is formed on the core layer. These three layers constitute an optical waveguide.
The optical multiplexing and demultiplexing circuit shown in FIG. 25 is composed of components 101 to 105, described below. That is, a reference numeral 101 denotes a plurality of input channel waveguides, a reference numeral 102 denotes an input slab waveguide, and a reference numeral 103 denotes an array waveguide that is substantially composed of a plurality of parallel waveguides of different lengths. Further, a reference numeral 104 denotes an output slab waveguide, and a reference numeral 105 denotes a plurality of output channel waveguides.
Such an array-waveguide grating type optical multiplexing and demultiplexing circuit can be used for both multiplexing and demultiplexing.
First, explanation will be given of functions of this circuit provided when it serves as an optical multiplexing circuit.
In an optical wavelength multiplexing communication method, a plurality of optical signals each having a predetermined wavelength and the multiplexed signal is transmitted through a single fiber. A wavelength of the transmitted optical signal is standardized and its method is recommended by the ITU (International Telecommunication Union). This recommendation specifies optical signals each having a same interval wavelength incrementing by a frequency of 100 or 50 GHz or the like. Common optical signals have a wavelength of one of these values or one-integer-th thereof (for example, a half or quarter thereof).
Such a plurality of optical signals having different wavelengths are input to the input ends of predetermined input channel waveguides 101, which are located at a facet of the substrate 100 via a plurality of optical waveguides or fibers. These optical signals pass through the input channel waveguides 101 and are guided to one facet of the input slab waveguide 102. Then, the guided signals are radiated from facet of the input channel waveguides 101 geometrically arranged at the above one facet of the input slab waveguide 102, and then input to the plurality of array waveguides 103 geometrically arranged at the other facet of the input slab waveguide 102.
The optical signals of the plural wavelength input to the array waveguides 103 travel through these waveguides, and have phase differences corresponding to differences in the lengths of the waveguides until they reach the other facet. The signals are then radiated in the output slab waveguide 104. The radiated optical signals interfere with each other and are condensed at the inlets of predetermined output channel waveguides 105. Then, the signals are multiplexed together and the multiplexed signal is output from a predetermined output channel waveguide 105. The multiplexed signal is provided to the exterior via an optical waveguide or an optical fiber connected to the output channel waveguide 105 at the corresponding facet of the substrate.
If the circuit is thus used as an optical multiplexing circuit, as many input channel waveguides 101 as wavelengths to be multiplexed as well as a single output channel waveguide 105 are commonly used.
In this description, one end of the arrayed-waveguide grating type optical multiplexing and demultiplexing circuit is defined as an input side, whereas the other end is defined as an output side. However, if the arrayed-waveguide grating type optical multiplexing and demultiplexing circuit is constructed to be symmetrical with respect to the array waveguides, then the input and output sides may be reversed. That is, if a multiplexed signal obtained by multiplexing optical signals of different wavelengths is input to the above described output channel waveguide from which the multiplexed output is obtained, then the optical signal travels in the direction opposite to that described above and is demultiplexed into predetermined input channel waveguides for output.
That is, if the arrayed-waveguide grating type optical multiplexing and demultiplexing circuit is used as an optical demultiplexing circuit, then it may be used in the opposite manner compared to the above described multiplexing circuit. That is, if this circuit is used as an optical demultiplexing circuit, a single input channel waveguide and as many output channel waveguides as wavelengths to be demultiplexed are used.
It is effective in forming a plurality of arrayed-waveguide grating type optical multiplexing and demultiplexing circuits on the same substrate as well as downsizing.
FIG. 26 schematically shows the input and output channel waveguides of the arrayed-waveguide grating type optical multiplexing and demultiplexing circuit. The characteristics of the multiplexing circuit will be described below.
Here, reference symbols #1, #2, . . . , #n denote input channel waveguides, and reference symbols *1, *2, . . . , *n denote output channel waveguides. If a plurality of optical signals each having an interval wavelength such as 100 or 50 GHz, as described previously, are sequentially input to the input channel waveguides #1, #2, . . . , #n so that the channel waveguides receive the optical signals of the corresponding wavelengths from short to long wavelengths or from long to short wavelengths, then the output channel waveguide from which a multiplexed output is obtained is denoted as *J.
FIG. 27A shows the optical transmitted wavelength characteristics of the input channel waveguide #1 and output channel waveguide *J in FIG. 26.
The axis of ordinates denotes optical transmittance, indicating that an upper part of the axis of ordinates is associated with a higher optical transmittance and weaker optical attenuation. That is, if a predetermined wavelength λ1 is input to the input transmission channel waveguides #1, optical signals having wavelengths near the wavelength λ1 are transmitted. If the wavelength changes from λ1 to a smaller or larger value, the optical transmittance decreases to increase transmission losses, causing wavelengths sufficiently distant from the wavelength λ1 to be very sharply attenuated.
The optical transmission wavelength characteristic is such that wavelengths near a center wavelength, which has the highest transmittance, generally exhibit a Gauss type, as shown in FIG. 27A. However, many efforts have been made to achieve a flat optical transmission wavelength characteristic within a predetermined wavelength range. The Gauss type will be described below by way of example.
Likewise, FIG. 27B shows the optical transmission wavelength characteristics of the input channel waveguide #2 and output channel waveguide *J in FIG. 26. Furthermore, FIG. 27C shows the synthesized optical transmission wavelength characteristics of the input channel waveguides #1, #2, . . . , #n and output channel waveguide *J in FIG. 26.
These figure indicate that if optical signals of the wavelengths λ1, λ2, . . . , λn are input to the input channel waveguides #1, #2, . . . , #n, respectively, they are multiplexed and output to the output channel waveguide *J.
Further, these figures also indicate an optical transmission wavelength characteristic observed in the following situation: if the circuit is used as a demultiplexing circuit as described previously, if a signal obtained by multiplexing optical signals of the wavelengths λ1, λ2, . . . , λn is input to the output channel waveguide *J, it is demultiplexed into n signals, which are then output to the input channel waveguides #1, #2, . . . , #n.
The above described characteristics of the optical multiplexing circuit are determined by both a waveguide width and a shape, or, both an installation position and an interval measured near the facet of the slab waveguide of the channel waveguides. These relations have been widely disclosed.
The operational principle and basic functions of the above described AWG optical multiplexing and demultiplexing circuit are disclosed in, for example, Japanese Patent No. 2599786, Japanese Patent Application Laid-open No. 5-313029 (1993), and other applications.
Optical transmitters require the optical wavelength multiplexing number to be increased in order to improve the transmission efficiency of an optical communication system. The wavelength multiplexing number n thus tends to be 16 to 32 channels or more. Accordingly, the interval between the predetermined optical signal wavelengths λ1, λ2, . . . , λn has decreased from the conventional value of 100 GHz to 50 GHz and will be smaller. Consequently, AWG optical multiplexing and demultiplexing circuits and peripheral circuits thereof must meet more strict specifications.
Optical oscillation circuits and electrooptic conversion circuits generally comprise laser diodes, and the stability of the oscillation wavelength of these circuits must meet such a strict specification. For example, the wavelength, which needed to be controlled to be equal to or less than ±10 GHz, now needs to be controlled to be equal to or less than ±5 GHz, and thus optical oscillation circuits must stabilize optical emission wavelength more precisely. To precisely control the optical wavelength, a method of controlling the temperature of laser diodes or the like is used.
Further, for example, Japanese Patent Application Laid-open Nos. 2000-65686 and 2000-78085 have proposed a circuit configuration for a wavelength multiplexing transmitter in which a receiver uses a demultiplexing circuit to demultiplex a received wavelength multiplexed signal into optical signals of different wavelengths and then these wavelengths are monitored and controlled for displacement from a predetermined wavelength. In these specifications, conventional optical multiplexing and demultiplexing circuits are used as simple multiplexing and demultiplexing functions, thereby requiring a peripheral circuit different from the optical multiplexing and multiplexing circuits to have a more complicated circuit configuration.
In short, in the prior art, the demultiplexing circuit is externally installed, and the deviations of the wavelengths of signals obtained by demultiplexing are detected by a filter or the like so as to control the wavelength of an electrooptic conversion circuit, that is, laser diodes, on the basis of the results of the detection. Consequently, the scale and size of the circuit must be increased.
In particular, as the wavelength multiplexing number increases to 16 to 32, implementation becomes more and more difficult and more and more costs are required.